The present invention relates to processing of a speech signal. Recently, various techniques have been known regarding processing of a speech signal. Speech-To-Text (STT), automatic speech recognition service, is designed to accept human speech data spoken to a microphone or a telephone directly in real situations. However, automatic speech recognition service sometimes accepts unique speech data that appears to be pre-processed before sending to STT. The STT service fails to decode this type of data well. Generally speaking, automatic speech recognition can decode clean data very well, and struggle decoding noisy data.